Diamond is a material having the greatest hardness and highest modulus of elasticity of all known materials. Furthermore, extremely pure diamond has additionally the highest thermal conductivity and the highest transmittance in the infrared spectrum. Thus, diamond is a material for which there are no comparable substitutes.
There are two general methods for synthesizing diamond using high temperature and high pressure. In one method, carbon material which is to be converted to diamond is mixed or brought into contact with a solvent metal such as iron, cobalt or nickel. Though the use of a stable high pressure and temperature, the carbon is converted into diamond under the action of the solvent metal. According to such a method, the solvent metal penetrates into the carbon material, whereby the carbon diffuses through the solvent metal, which is in the form of a thin film, to form a diamond. According to this method, diamond is spontaneously nucleated, and rapidly grows until it reaches a certain size. Considerable quantities of fine diamond powder have been synthesized by this method and applied to, e.g. abrasives. However, large diamond crystals of high quality cannot be synthesized by the aforementioned method.
On the other hand, a method of synthesizing large diamond crystals of good quality is disclosed in U.S. Pat. No. 3,297,407 issued Jan. 10, 1967 to R. H. Wentorf, Jr. Furthermore, U.S. Pat. No. 4,632,817 issued Dec. 30, 1986 to Yazu, provides a method for synthesizing a number of large diamond crystals of high quality simultaneously from a plurality of seed crystals. This method is generally called a temperature gradient method.
The present invention relates to using a diamond synthesized by the temperature gradient method.
Most diamond crystal dies are made of a natural diamond, whose (110) or (111) faces are polished. In addition, at least one face perpendicular to said faces is polished for purposes of observation and holing. After these preparations, a wire drawing hole, which is perpendicular to the (110) or (111) faces, is formed by a laser beam, electric discharge or ultrasonic grinding, while observing through the perpendicular polished face. Synthetic diamond crystals have been marketed and various attempts to use them for wire drawing have been carried out.
However, the tool life displayed by the prior art diamond dies varies over a wide range, causing them to be unreliable. Especially, the tool life of wire drawing dies made of natural single crystal diamonds can vary widely for the following reasons.
First, natural diamond crystal wire drawing dies have several faults. For example, it is said that a die having a wire drawing hole perpendicular to the (110) face, has generally the highest wear resistance and the longest life, as shown in Junkatsu (Lubrication), vol. 112, No. 11, 1967.
Second, it is difficult to judge precisely where the (110) face is because natural diamonds have been rounded, and have a variety of shapes, depending upon their degree of dissolution. As a result, it requires considerable skill to determine the (110) faces of a natural diamond being used as a wire drawing die. Therefore, most of the wire drawing holes are actually oriented in an unintended direction, making the tool life of the wire drawing die short.
Third, natural diamonds often have a soft portion and hard portion on one single crystal. As a result, the wire drawing hole partially wears during drawing, causing the wire drawing hole to deform into anything from a complete circle to a distorted circle. Such deformation also causes a shortened tool life.
Recently, a high quality synthetic diamond has been obtained by the temperature gradient method and is on the market. In spite of the many attempts to use the synthetic diamond for a wire drawing die, a high quality wire drawing die has not been obtained yet. The reasons for such unavailability are the following:
First, a synthesized or slightly polished idiomorphic diamond is generally used for the wire drawing die. In order to hold that diamond in a tool holder, a hexahedron diamond is usually employed. This diamond is mainly covered with (100) faces, with the wire drawing holes perpendicular to the (100) faces. However, a drawing hole of such direction has inferior wear resistance and a short tool life.
Second, some of the synthetic diamonds don't have a uniform concentration of nitrogen. This causes partial wear of the drawing hole.
Third, when the diamond is held in the tool holder using the surfaces of grown diamond or polished surfaces, a large stress is required to hold the diamond in place, because the friction coefficient between the diamond surfaces and holder is very low. As a result, the wire drawing die often cracks during drawing.
There are two well known methods for dividing a diamond: cutting and cleaving. In the former method, there are two ways to cut a diamond, one of which is represented by the following technique. A cutting blade, in which diamond particles are embedded, rotates at high speed, presses forward on the diamond and performs the cutting. The other technique involves thermal cutting, using a laser beam, for example.
In the cleaving method, a diamond is grooved by another diamond or a cutting blade and then a knife-shaped wedge is struck in the groove to cleave the diamond. This method is discussed on pages 411-412 of the Kesshokogaku Handbook. The cleaving method is thought to be the best method for dividing a diamond economically because it takes advantage of diamond's propensity to be easily cleaved parallel to the (111) faces. Moreover, the method does not cause a width loss (e.g., equal to the cutting blade), especially when compared to the cutting method. Similarly, it does not require a great deal of time to divide a diamond by this method.
However, the cleaving method is only used by commercial diamond cutters to divide large diamonds for jewelry and ornamental applications. This method is not used in industrial fields because: 1) it requires a high level of skill; 2) the possibility of destroying a high priced diamond is great; and 3) the probability of forming an uneven dividing plane is high. These limitations are discussed in Diamond (Sanyo Shuppan Bouekiu Ltd.), pages 216-218 and "Ceramics" 11 (1976) No. 6 page 520. In addition, it is impossible to divide a diamond to get several, thin and uniform plates which are indispensable for making heat sinks and wire drawing dies.
Based on the present inventors' research, the demands of cleaving by the prior art techniques have been clarified. Specifically, the angle between the groove and the cleavage plane (which forms the (111) faces) cannot be greater than 0.5.degree. in order to obtain an even face. However, it is impossible to groove with such accuracy using another diamond or a cutting blade because a diamond is hard enough that it only permits accuracy within 0.5". Also, the alignment of the groove has to be within 0.1 mm to achieve such cleavage. However, it is very difficult to groove with such accuracy using the prior art techniques.